Compound Diffraction Grating and Method of Manufacture

ABSTRACT

A method including forming a substrate to form a template which includes areas of high relief and areas of low relief; and forming a high refractive index diffraction grating in the template by adding high refractive index material to the template to form a continuous low relief surface The high refractive index material fills the areas of low relief and covers the areas of high relief of the template to form a high refractive index diffraction grating. The high refractive index diffraction grating includes the high refractive index material configured to have a low relief side corresponding to the continuous low relief surface and configured by the template to have a periodic side including areas of high relief and areas of low relief which periodically alternate in the first direction with the first periodicity and are interconnected by the high refractive index material.

TECHNOLOGICAL FIELD

Embodiments of the present invention relate to a diffraction grating andmethod of manufacture. In particular, they relate to a high refractiveindex diffraction grating and method of manufacture.

BACKGROUND

Transmissive diffraction gratings use interference of wave fronts from aperiodic structure to obtain diffraction effects.

A high refractive index diffraction grating has an interface betweenhigh refractive index material and low refractive index material thatincreases total internal reflection which can increase efficiency.

There is therefore a need for high refractive index diffractiongratings.

BRIEF SUMMARY

According to various, but not necessarily all, embodiments of theinvention there is provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful forunderstanding the brief description, reference will now be made by wayof example only to the accompanying drawings in which:

FIGS. 1A to 1E illustrate an example of a method for manufacturing, inhigh volumes, a high refractive index diffraction grating;

FIGS. 2A to 2C illustrate a method for forming a master mold;

FIG. 3 illustrates an example of an apparatus with a low-refractiveindex template between a high refractive index diffraction grating andthe high refractive index material;

FIG. 4 illustrates an apparatus with an attached substrate;

FIG. 5 illustrates an apparatus with the template removed;

FIG. 6 illustrates an example of the apparatus in use; and

FIG. 7 illustrates perspective view of a high refractive indexdiffraction grating.

DETAILED DESCRIPTION

The examples described below enable the manufacture in large quantitiesof very high quality, high efficiency, high refractive index diffractiongratings, which has not previously been possible.

FIGS. 1A to 1E illustrate an example of a method 100 for manufacturing,in high volumes, a high refractive index diffraction grating 40.

In this document, the term ‘high’ in relation to a refractive indexmeans a refractive index greater than 1.70.

In this document, the term ‘low’ in relation to a refractive index meansa refractive index less than 1.55.

The choice of particular high refractive index materials and lowrefractive index materials may depend on application. The refractiveindex difference between the high refractive index material and the lowrefractive index material may, for example, be controlled to achieved adesired efficiency at a desired diffraction order.

FIG. 1A illustrates deposition of material 31 in a master mold 20 toform a substrate 30. Material 31 is deposited to overfill parallelchannels 22 of the master mold 20 that repeat in a first direction 2with a first periodicity. The parallel channels 22 extend, in parallel,in a second direction 3 orthogonal to the first direction 2. At FIG. 1B,the substrate 30 has been formed in the master mold 20.

Where the material 31 contacts the side of the mold 20 that defines thechannels 22 of the mold, a periodic side 33 of the substrate 30 isformed. The periodic side 33 comprises interconnected areas of highrelief 34 (formed in the channels 22) and areas of low relief 35 (formedoutside the channels 22). The areas of high relief 34 and the areas oflow relief 35 periodically alternate in the first direction 2 with thefirst periodicity. The areas of high relief 34 and the areas of lowrelief extend, in parallel, in the second direction 3 orthogonal to thefirst direction 2.

The side 36 of the substrate 30 opposite the periodic side 33 has lowrelief. That is there is little or no modulation in relief from a baselevel over distances corresponding to the first periodicity.

The side 36 may, for example, be a planar surface. The planar surfacemay be flat or curved and the base level will then be respectively flator curved.

The substrate 30 is continuous. It extends without interruption in thefirst direction 2 and the second direction 3 and the areas of highrelief 34 and areas of low relief 35 are interconnected without gaps.

At FIG. 1C the substrate 30 is removed from the master mold 20 to form atemplate 32 for forming the high refractive index diffraction grating40.

The periodic side 33 comprises areas of high relief 34 and areas of lowrelief 35 that periodically alternate in the first direction 2 with thefirst periodicity. The areas of low relief 35 of the template 32 willform corresponding areas of high relief 44 of the high refractive indexdiffraction grating 40. The areas of high relief 34 of the template willform corresponding areas of low relief 45 of the high refractive indexdiffraction grating 40.

At FIG. 1D the high refractive index diffraction grating 40 is formed inthe template 32 by adding high refractive index material 42 to thetemplate 32.

As shown in FIG. 1E, the high refractive index material 42 is added tothe template 32 to over-fill the channels in the template 32 and forms acontinuous low relief surface 41. That is there is little or novariation in relief relative to a base level over distancescorresponding to the first periodicity. The low relief surface 41 may,for example, be a planar surface in which case the base level is planar.The planar surface may be flat or curved and the base level will then beflat or curved. The low relief surface 41 extends without interruption(gaps) in the first direction 2 and the second direction 3.

The high refractive index material 42 fills the areas of low relief 35of the template 32 and covers the areas of high relief 34 of thetemplate 32 to form a high refractive index diffraction grating 40 inthe template 32.

The high refractive index diffraction grating 40 therefore compriseshigh refractive index material 42 that is configured to have a lowrelief side 48 corresponding to the continuous low relief surface 41 andconfigured by the template 32 to have a periodic side 43 comprisingareas of high relief 44 and areas of low relief 45.

The areas of high relief 44 and the areas of low relief 45 of the highrefractive index material 42 periodically alternate in the firstdirection 2 with the first periodicity and are interconnected 46 by thehigh refractive index material 42 without gaps.

The areas of high relief 44 and the areas of low relief 45 extend, inparallel, in the second direction 3 orthogonal to the first direction 2.

The high refractive index diffraction grating 40 is therefore a block ofsolid material with a periodic side 43 comprising areas of high relief44 and areas of low relief 45 that alternate periodically in the firstdirection 2 and extend in parallel in the second direction 3.

In this example, the only discontinuities in the block of solid highrefractive index material 42 are surface discontinuities at the periodicside 43 where the surface is discontinuous because its reliefperiodically changes sharply, in this example, to form the highrefractive index diffraction grating 40.

Referring back to FIG. 1D, the high refractive index material 42 may beadded to the template 32 using atomic layer deposition (ALD). Atomiclayer deposition (ALD) is a thin film deposition technique that uses gasphase chemical processes sequentially and repetitively to slowly depositmaterial.

Referring back to FIG. 1D, the high refractive index material 42 may betitanium dioxide. This may be deposited, for example, using atomic layerdeposition. Other high refractive index materials may also be used, suchas SrTiO₃, LiNbO₃, BaTiO₃, Y₂O₃, MgO, Al₂O₃, Si₃N₄, SIO₂, TiN and VN

Referring back to FIG. 1A, the substrate 30 may, for example, be formedusing injection compression molding . Molding material 31, generallypreheated, may placed in an open, heated mold 20. The mold 30 is closedand pressure is applied to force the molding material 31 into contactwith the mold 20 until the molding material 31 has cured.

Referring back to FIG. 1A, the material 31 may, for example, comprise apolymer such as for example a polycarbonate or soluble polymer. Otherpolymers applicable for this method include but are not restricted topolyethylene napthalate (PEN), polyethylene terephthalate (PET),poly(methyl methacrylate) (PMMA), polyactide (PLA), nylon, etc.

In some examples, but not necessarily all examples, the template 32 maybe retained in the finished product and in these examples, the material31 forming the template 32 may be a low refractive index material. Theapparatus 10 formed at FIG. 1E then comprises: a low refractive indexdiffraction grating comprising low refractive index material 31configured to have a low relief side 36 and a periodic side 33, whereinthe periodic side 33 of the low refractive index material 31 iscontinuous comprising areas of high relief 34 and areas of low relief 35that periodically alternate in the first direction 2 with the firstperiodicity; and a high refractive index diffraction grating 40 incontact with the low refractive index diffraction grating, wherein thehigh refractive index diffraction grating 40 comprises high refractiveindex material 42 configured to have a low relief side 48 and a periodicside 43 comprising areas of high relief 44 and areas of low relief 45,wherein the areas of high relief 44 and the areas of low relief 45 ofthe high refractive index material 42 periodically alternate in thefirst direction 2 with the first periodicity and are interconnected 46by the high refractive index material 42.

The areas of high relief 44 of the high refractive index diffractiongrating 40 contact areas of low relief 35 of the low refractive indexdiffraction grating and areas of low relief 45 of the high refractiveindex diffraction grating 40 contact areas of high relief 34 of the lowrefractive index diffraction grating.

Where the apparatus 10 illustrated in FIG. 1E is a final product, thelow relief side 48 of the high refractive index material 42 defines afirst exterior surface of the apparatus 10 and the low relief side 36 ofthe low refractive index material 32 defines a second exterior surfaceof the apparatus 10 opposite the first exterior surface.

FIG. 3 illustrates an example of how the apparatus 10 may be augmented.A layer 49 of high refractive index material 42 is formed on the lowrelief side 36 of the template 32 that opposes the periodic side 33 ofthe template 32. The template 32 is formed from low-refractive indexmaterial and the layer 49 of high refractive index material 42 formed onthe low relief side 36 of the template 32 enhances total internalreflection of light within the template 32. The apparatus 10 illustratedin FIG. 3 may be a final product.

FIGS. 2A to 2C illustrate a method 110 for forming a master mold 20.

As illustrated in FIG. 2A, a resist layer 64 is formed over a substrate60. The resist layer 64 may be selectively patterned 62 and selectivelyremoved to form a patterned resist 64 as illustrated in FIG. 2B. In thisexample, the patterned resist comprises stripes of resist material thatare evenly separated in the first direction 2 and extend, in parallel,in the second direction 3. The resist 64 may be a lithographicallypatterned photoresist layer or a layer of resist (for example, Cr)patterned using a lithographically patterned photoresist layer.

As illustrated in FIG. 2C, material is etched 66 from the substrate 60using the patterned resist 64 as a mask. The etching creates the mastermask 20 that has channels 22. The channels 22 are evenly spaced in thefirst direction 2 with the first periodicity and the channels runparallel to each other in the second direction 3.

The patterning of the resist 64 may, for example, be achieved usinge-beam or optical lithography.

The selective removal of the substrate 60 to form the mold 20 may beachieved using, for example, reactive ion-beam etching.

Although a particular cross-sectional profile has been illustrated, itshould be appreciated that many other profiles may be used to make mold20 used to manufacture high refractive index diffraction gratings 40having different profiles.

In this example, the channels 22 are slanted. The side walls of thechannels 22 form an acute angle with a top surface of the master mold20. In other examples, the channels 22 form a 90 degree angle with a topsurface of the master mold 20.

In this example, the side walls of the channels 22 are parallel and eachof the channels has a base that forms one of the areas of low relief 45.In other examples, the sidewalls of a channel may not be parallel andthey may meet at an edge to define of area of low relief 45.

The profile of the master mold 20 is formed in negative image in thetemplate 32 and in positive image in the high refractive indexdiffraction grating 40

For example, the master mold 20 and the high refractive indexdiffraction grating 40 may both have alternative periodic profiles suchas binary, sinusoidal, step-function or blazed. Some of these profilesare discontinuous with four sharp transitions per period (binary,step-function), some are discontinuous with two sharp transitions perperiod (blazed) and some are continuous with no sharp transitions perperiod (sinusoidal, arbitrary periodic, continuously changing reliefprofiles)FIG. 4 illustrates a further process that may be applied to theapparatus 10 illustrated in FIG. 1E.

A substrate 50 is added to the low relief side 48 of the high refractiveindex diffraction grating 40. The substrate 50 may be a substrate 50 ofhigh refractive index glass and it may be attached to the low reliefside 48 of the high refractive index diffraction grating 40 usingoptically clear adhesive.

The apparatus 10 illustrated in FIG. 4 may be a final product.

In some examples, but not necessarily all examples, the template 32 maybe retained in the final product and in these examples, the material 31forming the template 32 may be a low refractive index material 31. Theapparatus 10 formed at FIG. 4 then comprises: a low refractive indexdiffraction grating; and a high refractive index diffraction grating 40in contact with the low refractive index diffraction grating.

As previously described and as shown in FIG. 1E, the low refractiveindex diffraction grating comprises low refractive index material 31configured to have a low relief side 36 and a periodic side 33. Theperiodic side 33 of the low refractive index material 31 is continuouscomprising areas of high relief 34 and areas of low relief 35 thatperiodically alternate in the first direction 2 with the firstperiodicity.

As previously described and as shown in FIG. 1E, the high refractiveindex diffraction grating 40 comprises high refractive index material 42configured to have a low relief side 48 and a periodic side 43comprising areas of high relief 44 and areas of low relief 45. The areasof high relief 44 and the areas of low relief 45 of the high refractiveindex material 42 periodically alternate in the first direction 2 withthe first periodicity and are interconnected 46 by the high refractiveindex material 42.

The areas of high relief 44 of the high refractive index diffractiongrating 40 contact areas of low relief 35 of the low refractive indexdiffraction grating and areas of low relief 45 of the high refractiveindex diffraction grating 40 contact areas of high relief 34 of the lowrefractive index diffraction grating.

Where the apparatus 10 is a final product, the low relief side 36 of thelow refractive index material 31 defines a first exterior surface of theapparatus 10 and the substrate 50 defines a second exterior surface ofthe apparatus 10 opposite the first exterior surface.

FIG. 5 illustrates a further process that may be applied to theapparatus 10 illustrated in FIG. 4.

The apparatus 10 as illustrated in FIG. 4, may be modified by removingthe template 32 from the apparatus 10 leaving the combination of highrefractive index diffraction grating 40 and substrate 50, as illustratedin FIG. 5. The template 32 is not retained but is sacrificial and isabsent exposing the periodic side 43 of the high index diffractiongrating 40 to a low refractive index atmosphere 70 such as the ambientair.

As the template 32 is not retained, it need not be a low refractiveindex template.

The apparatus 10 illustrated in FIG. 5 may be a final product.

The apparatus 10 formed at FIG. 5 comprises: a high refractive indexdiffraction grating 40 comprising high refractive index material 42configured to have a low relief side 48 and a periodic side 43comprising areas of high relief 44 and areas of low relief 55. The areasof high relief 44 and the areas of low relief 45 of the high refractiveindex material 42 periodically alternate in the first direction 2 withthe first periodicity and are interconnected 46 by the high refractiveindex material 42. The low relief side 48 of the high refractive indexmaterial 42 contacts second high diffraction index material of thesubstrate 50.

FIG. 6 illustrates an example of how any of the apparatus 10 previouslydescribed may be used. Other uses are possible. In this example, thehigh index refractive grating 40 is used as a transmissive diffractiongrating. The high refractive index diffraction grating 40 is used as anexit pupil diffraction grating 82 of an exit pupil expander 80 and/or asan entrance pupil diffraction grating 84 of the exit pupil expander 80.The entrance pupil diffraction grating is shorter than the exit pupiland they are interconnected via a light guide 86. Light 83 enters theentrance pupil diffraction grating 84 and then exits the exit pupildiffraction grating 82.

Other uses of the apparatus 10 are where large area, precisiondiffraction gratings are required, for example, as a backlight gratingarray for a display.

FIG. 7 illustrates in more detail an example of a geometry of oneexample of a high refractive index diffraction grating 40.

As previously described the cross-section profile of the high refractiveindex diffraction grating 40, while periodic, may take different forms.

The periodic side 43 of the high refractive index material 42 isperiodic only in the first direction 2. It is continuous and hasunchanging relief in the second direction 3 orthogonal to firstdirection 2. The high refractive index material 42 is discontinuous in athird direction 4 orthogonal to the first direction 2 and the seconddirection 3.

The areas of high relief 44 and areas of low relief 45 extend parallelto the second direction 3 orthogonal to the first direction 2. The areasof low relief 45 form parallel channels.

In this example, but not necessarily all examples, the areas of highrelief 44 form planar parallel surfaces and the areas of low relief 45form planar parallel surfaces. This is not the case for example in ablazed diffraction grating.

In this example, but not necessarily all examples, the areas of highrelief 44 form planar parallel surfaces that share a common upper planeand the areas of low relief 45 form planar parallel surfaces that sharea common lower plane parallel to the common upper plane. This is not thecase for example in a blazed diffraction grating.

In this example, but not necessarily all examples, the interconnectingareas 46 of the high index material between the adjacent areas of highrelief 44 and low relief 45 are at a common angle to the first direction2. This is not the case for example in a blazed diffraction grating.

The term ‘periodic’ in the first direction 2 means that in the firstdirection the feature of interest (e.g. high relief area 44 and/or lowrelief area 45) repeats every distance d, where d is the period.

In this example, but not necessarily all examples, the areas of highrelief 44 and the areas of low relief 45 have the same length in thefirst direction 2. The high refractive index diffraction grating 40 is abinary diffraction grating.

Over relatively long (mm scale) distance, the periodicity and/or theheight of the relief may slowly change to allow for modulation of eitherthe period or height of the grating profile.

The blocks illustrated in the Figs may represent steps in a method . Theillustration of a particular order to the blocks does not necessarilyimply that there is a required or preferred order for the blocks and theorder and arrangement of the block may be varied. Furthermore, it may bepossible for some blocks to be omitted.

As used here ‘module’ refers to a unit or apparatus that excludescertain parts/components that would be added by an end manufacturer or auser. The apparatus 10 may be a module.

The term ‘comprise’ is used in this document with an inclusive not anexclusive meaning. That is any reference to X comprising Y indicatesthat X may comprise only one Y or may comprise more than one Y. If it isintended to use ‘comprise’ with an exclusive meaning then it will bemade clear in the context by referring to “comprising only one..” or byusing “consisting”.

In this brief description, reference has been made to various examples.The description of features or functions in relation to an exampleindicates that those features or functions are present in that example.The use of the term ‘example’ or ‘for example’ or ‘may’ in the textdenotes, whether explicitly stated or not, that such features orfunctions are present in at least the described example, whetherdescribed as an example or not, and that they can be, but are notnecessarily, present in some of or all other examples. Thus ‘example’,‘for example’ or ‘may’ refers to a particular instance in a class ofexamples. A property of the instance can be a property of only thatinstance or a property of the class or a property of a sub-class of theclass that includes some but not all of the instances in the class. Itis therefore implicitly disclosed that a feature described withreference to one example but not with reference to another example, canwhere possible be used in that other example but does not necessarilyhave to be used in that other example.

Although embodiments of the present invention have been described in thepreceding paragraphs with reference to various examples, it should beappreciated that modifications to the examples given can be made withoutdeparting from the scope of the invention as claimed.

Features described in the preceding description may be used incombinations other than the combinations explicitly described.

Although functions have been described with reference to certainfeatures, those functions may be performable by other features whetherdescribed or not.

Although features have been described with reference to certainembodiments, those features may also be present in other embodimentswhether described or not.

Whilst endeavoring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

I/We claim: 1-13. (canceled)
 14. An apparatus comprising: a highrefractive index diffraction grating comprising high refractive indexmaterial configured to have a planar side and a periodic side, theperiodic side extending in a first direction and comprising areas ofhigh relief and areas of low relief, wherein the areas of high reliefand the areas of low relief of the high refractive index materialperiodically alternate in the first direction with the first periodicityand are interconnected by the high refractive index material to form aplurality of channels, the channels having parallel side walls that areslanted to form an acute angle relative to the first direction; andwherein the planar side of the high refractive index material contacts asecond high refractive index material with a refractive index greaterthan 1.70, wherein the second high refractive index material defines anexterior surface of the apparatus.
 15. (canceled)
 16. An apparatus asclaimed in claim 14, wherein the low relief side of the high refractiveindex material defines an exterior surface of the apparatus; or whereinthe low relief side of the high refractive index material contacts highrefractive index material defining an exterior surface of the apparatus;and/or wherein the low relief side of the low refractive index materialcontacts high refractive index material.
 17. An apparatus as claimed inclaim 14 comprised in an exit pupil expander .
 18. A method comprising:forming a substrate in a master mold having a top surface and aplurality of channels in the top surface, the channels having parallelside walls that are slanted to form an acute angle relative to the topsurface; removing the substrate from the master mold to form a templatehaving a periodic side that is continuous and comprises areas of highrelief and areas of low relief that periodically alternate in a firstdirection with a first periodicity; and forming a high refractive indexdiffraction grating in the template by adding high refractive indexmaterial to the template to form a continuous planar surface , the highrefractive index material filling the areas of low relief of thetemplate and covering the areas of high relief of the template to form ahigh refractive index diffraction grating in the template , wherein thehigh refractive index diffraction grating comprises the high refractiveindex material configured to have a planar side corresponding to thecontinuous planar surface and configured by the template to have aperiodic side comprising areas of high relief and areas of low relief,wherein the areas of high relief and the areas of low relief of the highrefractive index material periodically alternate in the first directionwith the first periodicity and are interconnected by the high refractiveindex material , wherein the method comprises forming a high refractiveindex diffraction grating in the template by adding high refractiveindex material to the template.
 19. A method as claimed in claim 18,comprising forming a high refractive index diffraction grating in thetemplate by adding high refractive index material to the template usingatomic layer deposition.
 20. A method as claimed in claim 18, comprisingforming the high refractive index diffraction grating in the templateusing titanium dioxide, SrTiO₃, LiNbO₃, BaTiO₃, Y₂O₃, MgO, AI₂O₃, Si₃N₄,SiO₂, TiN or VN.
 21. A method as claimed in claim 18, comprising forminga layer of high refractive index material on a side of the template thatopposes the periodic side of the template.
 22. A method as claimed inclaim 18, wherein the template comprises polymer.
 23. A method asclaimed in claim 18, comprising forming the template using injectioncompression molding of the high refractive index material.
 24. A methodas claimed in claim 18, comprising creating a master mold wherein themaster mold has a plurality of parallel channels.
 25. A method asclaimed in claim 18, comprising adding a substrate to the planar side ofthe high refractive index diffraction grating.
 26. A method as claimedin claim 25, wherein the substrate comprises high refractive indexglass.
 27. A method as claimed in claim 18, wherein the low refractiveindex template is retained in a direct product of the method or whereinthe low refractive index template is not retained and is absent from adirect product of the method exposing the periodic side of the highindex diffraction grating to a low refractive index atmosphere.
 28. Amethod as claimed in claim 18, comprising forming as a direct product anapparatus as claimed in any of following claim 14.